This invention relates to hollow microspheres with essentially siliceous shells. In particular, the invention comprises hollow microspheres of a new composition that have improved physical characteristics and a low surface energy and methods for preparing them.
Essentially siliceous hollow microspheres are well known articles of commerce which can be used as fillers for various polymer systems, explosives, syntactic foams, etc. These products are made in a number of ways, the most common being passing certain solid particles of glass-forming materials through spherulizing furnaces or spray drying solutions of film-forming materials. The former results in a fused surface, a modified surface energy and somewhat lower chemical reactivity. The spray drying methods which are disclosed in U.S. Pat. Nos. 3,699,050; 3,794,503; 3,796,777; 3,838,998 and 3,888,957 appear to form microspheres that are more reactive. While the microspheres exit the spray dryer as discrete spheres, their high energy, polar and reactive surfaces quickly result in particle agglomeration and caking. These properties often result in breakage during processing and prevent the microspheres from settling well and expelling air from containers. Maintaining the hollow integrity of these products is an important consideration in their value. Solid flow conditioning agents are usually added to the microspheres after spray drying in an effort to mitigate these problems. These additives are not totally effective, and detract from product quality because of their higher density and their porous nature. These additives adhere to the microspheres so that their smooth outer surfaces become irregular. It is apparent that changing the composition, reducing the reactivity and lowering the surface energy of these microspheres as they are forming results in important product improvements.
One method of modifying the surfaces of some siliceous fillers has been to coat them with various materials such as organosilanes after the materials have been manufactured. U.S. Pat. Nos. 3,915,735 and 4,141,751 teach the silane coating of microcrystalline silica by pouring or spraying the silane onto a tumbling bed of the silica. U.S. Pat. No. 4,134,848 teaches slurrying a hollow microsphere with a solution containing a silane to form a coated microsphere. There are, of course, numerous teachings in the literature to silane coating various substrates, but the references cited herein appear to be the most relevant. This solution to the problem is an "additive approach" and not completely satisfactory since it requires a process step after the microsphere has been produced. This post-coating approach is also flawed because of the difficulty in obtaining uniform coatings while maintaining processing conditions that do not damage or destroy the microspheres.